ARM: mxs: Fix build failure by using wrong constant for .sync attribute

This fixes the build failure due wrong constant in .sync attribute in
new board code.

-               .sync           = FB_SYNC_DATA_ENABLE_HIGH_ACT
+               .sync           = MXSFB_SYNC_DATA_ENABLE_HIGH_ACT

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>